1. Field of the Invention
The present invention relates to a transmission apparatus and a communication system. To be more particular, embodiments of the present invention are intended to realize high-speed transmission based on IQ quadrature axis by shifting a first input point for inputting a first transmission signal into a waveguide and a second input point for inputting a second transmission signal into the waveguide by a distance that gives a predetermined phase difference between the first transmission signal and the second transmission signal.
2. Description of the Related Art
These days, researches into high-speed transmission technologies based on high-frequency bands such as millimeter waves have been vigorously under way in order to attain low power dissipating and low cost signal transmission based on small circuit scales using CMOS (Complementary Metal Oxide Semiconductor) technologies. This is because it is possible to realize this signal transmission by configuring digital circuit module and a millimeter wave RF front-end module with a single chip by use of CMOS processes suitable for mass production. Related-art high-speed baseband signal transmission apparatuses are each configured by a board low in dielectric loss, a first part and a second part installed on this board, and a waveguide providing connection between these first and second parts. By reducing the interference between these first and second parts on the board, the high-speed signal transmission between these first and second parts on the board is realized.
It should be noted here that, in the above-mentioned high-speed baseband signal transmission apparatus and so on, a phase modulation scheme based on the quadrature IQ axis has been in wide use in order to achieve faster transmission rates. In phase modulation, the phase of carrier wave is discretely changed in accordance with a digital code in digital transmission for example, executing digital code mapping for phase component θ given by equation (1) below.S(t)=A(t)cos(2πfc+θ(t))  (1)
In the equation above, A(t) denotes amplitude and θ(t) denotes phase.
In the case of QPSK (Quadrature Phase Shift Keying) modulation that is one of phase modulation schemes, for example, QPSK modulation can be realized by expressing a 2-bit digital code by one phase. FIG. 16 shows an example of mapping of digital code and phase θ. FIG. 17 shows how each digital code is mapped on a complex baseband signal. Thus, a technology for enhancing high-speed transmission by imposing multi-bit information on one piece of phase information has been used for long. Other multi-bit transmission technologies include QAM (Quadrature Amplitude Modulation) transmission for example that uses both amplitude and phase is popular these days.
However, the execution of these transmission schemes desires a 90-degree phase shifter or the like for orthogonalizing I-axis and Q-axis on a complex plane, for example, and, when multi-bit transmission is attempted, this orthogonalization has to be realized with a high precision. Hence, a variety of accuracy compensation methods have been proposed so far.
For example, Japanese Patent Laid-open No. Hei 10-70582 (page 5, FIG. 1), which is hereinafter referred to as Patent Document 1, discloses a transmitting device having a compensating circuit configured, in consideration of the accuracy of an quadrature modulating circuit, to suppress a carrier wave leak below a desired value by increasing or decreasing a direct current bias so that a voltage detected by a wave detector goes below a predetermined level.
Japanese Patent Laid-open No. 2007-150646 (page 8, FIG. 4), which is hereinafter referred to as Patent Document 2, discloses a wireless communication apparatus configured to switch to the BPSK (Binary Phase Shift Keying) scheme if the accuracy is poor and IQ orthogonality is unnecessary and switch to the QPSK scheme if the accuracy is good, thereby assuring the maintenance of the transmission channel rather than the maintenance of IQ quadrature accuracy.